Electromyography

Motor Unit Configuration and Amplitude

Amplitudes of motor unit action potentials (MUAPs) are lower in infants, with amplitudes ranging from 150 microvolts to approximately 2,000 microvolts.

7.4

Normal Sensory Conduction Velocities (m/sec)

Data are presented as means ± SD

*Velocities based on peak latencies for Cai and Zhang (9); others based on onset latencies

D-W = Finger to wrist using ring electrodes with orthodromic stimulation

D-E = Finger to elbow using ring electrodes with orthodromic stimulation

D2 = Index finger for median; D3 = middle finger for median; D5 = fifth finger for ulnar

Sural nerve studies use antidromic with recording electrodes behind the lateral malleolus with stimulus delivered at 6 cm to 14 cm above the malleolus as specified.

1,000 microvolts in 0- to 3-year-old children are rare (24,25). In infants, motor unit action potentials are usually biphasic or triphasic.

Motor Unit Duration

Infantile motor unit action potentials are often shorter in duration. DeCarmo (24) found newborn infants to exhibit durations 17% to 26% shorter than those seen in adults. Durations of motor unit action potentials are often shorter than 5 milliseconds in infants.

Motor Unit Recruitment

In very young infants and children, it is difficult to assess strength of voluntary contraction and deter­mine when the interference pattern is full. In general, as strength of voluntary contraction increases, there is an increase in motor unit action potentials recruited. However, the recruitment pattern in infants may be disordered and chaotic. As with adults, the recruit­ment frequency, defined as the firing rate of a MUAP when a different MUAP first appears, with gradually increasing strength of voluntary contraction, is helpful

7.5

Normal Sensory Nerve Action Potential (SNAP) Amplitudes in Children (μV)

Data are presented as means ± SD

Amplitudes are determined peak-to-peak from positive-to-negative peak of the SNAP

D-W = Finger to wrist using ring electrodes with orthodromic stimulation

D2 = Index finger for median; D3 = middle finger for median; D5 = fifth finger for ulnar

Sural nerve studies used antidromic stimulation with recording electrodes behind the lateral malleolus with stimulus delivered at 6 cm to 14 cm above the malleolus as specified.

in differentiating a myopathic process (lower recruit­ment frequency values) from a neuropathic process (higher recruitment frequencies after greater than 20-25 Hz). An example of neuropathic recruitment is shown in Figure 7.1.